(i) Field of the Invention
The present invention relates to a tap device of a cable broadcasting system in which a part of transmission signal running through a transmission line leading to a terminal side from a center equipment is branched and transmitted to a plurality of subscriber terminal devices, particularly to a tap device of a cable broadcasting system in which it can be switched on the side of the center equipment whether the transmission signals are to be transmitted or not to the terminal devices.
(ii) Description of the Related Art
In a conventional cable broadcasting system such as CATV system in which broadcasting signals such as television signals are transmitted to a subscriber""s terminal device via one transmission line formed of a coaxial cable, and the like, the transmission line is provided with a tap device, a so-called tap-off, for taking in signals in order to branch the broadcasting signals from the transmission line and leading the signals into the subscriber""s terminal device.
Moreover, in a known tap device, it can easily be switched according to a subscriber""s request or the like whether or not the broadcasting signals are distributed to the subscriber""s terminal device.
Specifically, the tap device is provided with a relay, a so-called high-frequency relay, disposed in a signal path between a directional coupler for branching a part of the broadcasting signals from the transmission line and a tap output terminal for outputting the branched broadcasting signals toward the subscriber""s terminal device, in which high-frequency broadcasting signals can be passed with a reduced loss. By operating the relay in response to a command signal transmitted from the center equipment via the transmission line, the connected/disconnected state of the signal path, in other words, whether or not the broadcasting signals are outputted to the terminal device, can easily be switched on the side of the center equipment.
Additionally, since this type of tap device is usually disposed outdoors, a directional coupler, a relay operating control circuit, and the like are all incorporated in a conductive sealed housing.
Therefore, a noise generated by the switching of the relay, or a noise from the relay operating control circuit easily enters the broadcasting signal path for leading the broadcasting signals branched from the transmission line by the directional coupler to the tap output terminal, and the broadcasting signals outputted from the tap output terminal is deteriorated in some cases.
On the other hand, to solve such problems, various shield materials may be used to shield the signal path of the broadcasting signals.
However, there is a tap device in which not only the broadcasting signals flowing through the transmission line are branched but also the branched broadcasting signals are further split and outputted from a plurality of tap output terminals, respectively. In this tap device, the broadcasting signal paths of the same number as that of the tap output terminals are formed, and relays are disposed on the respective paths. Therefore, it is difficult to shield each signal path using a shield material in order to prevent the leakage of the high-frequency broadcasting signals in a limited space.
Specifically, since the tap devices are arranged in series on the transmission line of the CATV system or the like, the size reduction is required, and the enlargement of the volume of the sealed housing is limited. On the other hand, in the tap device provided with a plurality of tap output terminals, the number of components such as relays to be incorporated in the sealed housing is increased. As a result, in the tap device provided with a plurality of tap output terminals, the housing has therein little space left for housing the shield material, and it is difficult to shield each signal path using the shield material.
Moreover, when a plurality of relays are arranged in the housing, in a circuit substrate with these relays mounted thereon, the leakage of high-frequency broadcasting signals is caused by the capacity coupling between wiring patterns forming the broadcasting signal paths, or between the wiring patterns and the relays. In this case, even when some of the relays are turned off, the broadcasting signals are disadvantageously outputted from the corresponding tap output terminals.
Wherefore, an object of the present invention is to provide a tap device of a cable broadcasting system for branching a part of transmission signals running through a transmission line and selectively transmitting the signals to subscriber terminal devices via a plurality of tap output terminals, in which a signal path for leading the transmission signals to each tap output terminal can easily and securely be shielded.
To achieve this and other objects, according to one aspect of the present invention, there is provided a tap device which includes a first housing member provided with a pair of input and output terminals for connecting the tap device to a transmission line leading to the terminal side from the center equipment of a cable broadcasting system, and a second housing member provided with a plurality of tap output terminals for branching and transmitting transmission signals running through the transmission line of the cable broadcasting system to subscriber terminal devices. These housing members have openings which can be connected with each other. By combining the housing members via the openings, a sealed housing is formed.
Moreover, the following three types of circuit substrates are disposed in the sealed housing formed of the first and second housing members:
(1) a directional coupler substrate provided with a directional coupler for connecting the input and output terminals and passing the transmission signals between these terminals while branching a part of the transmission signals;
(2) a splitter substrate provided with a splitter for splitting the transmission signals branched by the directional coupler and transmitting the signals to the tap output terminals, and a plurality of relays for connecting/disconnecting signal paths leading to the tap output terminals from the splitter; and
(3) a control circuit substrate provided with a receiving circuit for taking in the transmission signals branched by the directional coupler and receiving a command signal superimposed onto the transmission signals and directed to the tap device from the center equipment, and a control circuit for operating the relays in response to the command signal received by the receiving circuit to control the connected/disconnected state of each signal path in accordance with a command from the center equipment.
The splitter substrate, the control circuit substrate, and the directional coupler substrate are arranged and housed in this order from the second housing member.
The relays are mounted on the surface of the splitter substrate facing the second housing member. Furthermore, grounding patterns are formed to surround the relays and the connecting portions of the relays and the tap output terminals on the side of the second housing member. Disposed on the surface of the second housing member facing the splitter substrate are partition walls which surround the tap output terminals, abut the grounding patterns of the splitter substrate, and separate the relays and the connection portions of the relays and the tap output terminals.
Therefore, when these substrates are housed in the sealed housing, the signal path leading to the tap output terminal from each relay of the splitter substrate is shielded by the grounding pattern formed on the surface of the splitter substrate on the side of the second housing member and the partition wall disposed on the second housing member. Therefore, the noise generated on the side of the control circuit substrate is prevented from entering the signal path leading to the tap output terminal from each relay of the splitter substrate, and the transmission signals outputted from each tap output terminal is prevented from being deteriorated.
Moreover, the portion between the signal paths leading to the tap output terminals from the relays, or the portion between each signal path and the other relay can be shielded by the grounding pattern of the splitter substrate and the partition wall on the side of the second housing member. Therefore, the transmission signals can be prevented from leaking toward the tap output terminal which is prohibited from outputting the transmission signals by turning off the relay, from the tap output terminal which is permitted to output the transmission signals by turning on the relay.
Moreover, in the present invention, since the partition wall disposed on the second housing member is used for the shielding, the device constitution can be simpler as compared with the conventional device provided with a separate shield material. Furthermore, even when the number of tap output terminals is increased, the number of shielding components is not increased, so that the size of the device can easily be reduced.
In the tap device of the present invention, a wiring pattern for transmission signal input to each relay and a wiring pattern for transmission signal output via each relay are formed on the surface of the splitter substrate facing the control circuit substrate, a grounding pattern is formed between these wiring patterns, and a shield cover is disposed between the splitter substrate and the control circuit substrate.
Furthermore, the shield cover is provided with a plate portion which can cover the wiring patterns leading to the relays from the splitter, and a partition wall portion disposed on the surface of the plate portion on the side of the splitter substrate for abutting the grounding pattern of the splitter substrate to separate the wiring pattern for the transmission signal input to each relay and the wiring pattern for the transmission signals output via each relay.
Therefore, in the tap device of the present invention, since the wiring pattern formed on the surface of the splitter substrate facing the control circuit substrate is covered with the shield cover, the noise generated by the operations of the receiving circuit and control circuit mounted on the control circuit substrate is prevented from entering the signal path for passing the transmission signals on the side of the splitter substrate, so that the transmission signal transmitted to the subscriber terminal device from the tap output terminal can more securely be prevented from being deteriorated.
Moreover, in the splitter substrate, the grounding pattern is formed between the wiring pattern for the transmission signal input to each relay and the wiring pattern for the transmission signal output via each relay, and the shield cover is provided with the partition wall portion which abuts the grounding pattern to separate the wiring patterns. Therefore, when the relay is in an off state, the transmission signals can be prevented from leaking to the output wiring pattern from the input wiring pattern. According to the present invention, the signal isolation degree during the relay off is enhanced, and the transmission signal can more securely be prevented from leaking to the subscriber terminal device from the tap output terminal.
Additionally, in the tap device of the present invention, screw holes are made in the splitter substrate and the shield cover to simultaneously fix the splitter substrate and the shield cover to the second housing member. The splitter substrate and the shield cover are simultaneously screwed to a substrate fixing portion formed on the second housing member by inserting common substrate fixing screws into the screw holes.
Therefore, in the tap device of the present invention, when the splitter substrate and the shield cover are incorporated in the sealed housing, simply by fixing these components to the second housing member with the common screw, the components can simultaneously be positioned and fixed into the housing, so that the assembly operation of the tap device can efficiently be performed.
Additionally, in the tap device of the present invention, the substrate fixing portion for fixing the control circuit substrate is formed on the shield cover. When the control circuit substrate is fixed into the housing, the screw is inserted to the screw hole made in the position of the control circuit substrate corresponding to the substrate fixing portion, and the control circuit substrate is screwed to the substrate fixing portion formed on the shield cover.
Therefore, according to the tap device of the present invention, the number of components for housing/fixing the control circuit substrate in the closed housing is decreased, and the operation efficiency of the assembling of the tap device can be enhanced.
Specifically, in order to dispose the control circuit substrate on the splitter substrate fixed to the second housing member, a spacer needs to be disposed therebetween to prevent the substrates from contacting each other. In the present invention, however, since the shield cover functions as the spacer, a separate spacer does not need to be disposed, and the number of components can be decreased. Moreover, since the shield cover functions not only as the spacer but also as a member for fixing the control circuit substrate to the second housing member, a separate fixing member does not need to be disposed, thereby the number of components is decreased. As described above, since neither a separate spacer nor a separate fixing member is necessary, the number of processes for assembling the tap device can be decreased, and the operation efficiency can be enhanced.
On the other hand, in the tap device of the present invention, a base for fixing the directional coupler substrate is mounted on the control circuit substrate, and the directional coupler substrate is fixed to the control circuit substrate via a resin case which has an attaching portion to the base.
Therefore, according to the tap device of the present invention, the procedure for assembling the tap device includes first screwing/fixing the splitter substrate and the shield cover simultaneously to the second housing member, disposing and screwing the control circuit substrate onto the shield cover, and further fixing the directional coupler substrate. The circuit substrates are fixed to the second housing member in order by the procedure. Subsequently, the first housing member is attached to the second housing member from above, and the housing is sealed. Thus, the tap device of the present invention can be assembled extremely easily.
Additionally, the directional coupler is mounted on the directional coupler substrate. The directional coupler connects the input and output terminals disposed on the first housing member, passes the transmission signals between the terminals and branches a part of the transmission signals. When the directional coupler substrate is fixed onto the control circuit substrate fixed on the second housing member as described above, the input and output terminals on the first housing member need to be connected to the directional coupler before the first housing member is attached over the second housing member with the circuit substrates mounted thereon. To easily perform the connecting operation, connectors which can be engaged with each other are disposed on the directional coupler substrate and the first housing member. When the opening of the first housing member is connected to the opening of the second housing member, the input and output terminals on the first housing member and the directional coupler can simultaneously be connected via the connectors.